The gripping force F of a tire can be given by the product of the frictional coefficient .mu. between the tire and the road surface and the vertical contact load W acting on the tire contact surface (F=.mu.W). In other words, the tire gripping force which plays an important role in the handling of the vehicle is proportional to the magnitude of the tire contact load for a given road condition.
It is well known that the handling of a vehicle is severely impaired when a road wheel thereof is locked up when braking. In particular, on a road surface such as a frozen road surface and a gravel road surface which could significantly reduce the gripping force of the tire, it is important to apply as large a braking force to the wheel as possible, but, without locking the wheel.
The road gripping force of a tire is important also when accelerating a vehicle. When a vehicle is excessively accelerated for a given road condition, the tires start slipping, and not only a desired acceleration is prevented from being achieved, but also the lateral stability of the vehicle may be lost. By noting this problem, it has been proposed to control the traction force of each driven wheel so that the slip ratio of the wheel may be kept within a limit, and a maximum available traction may be obtained at all times. The traction control system is designed to carry out such a control action. However, the conventional traction control system was not able to increase the magnitude of the available traction, and simply reduces the torque transmitted to the wheels so as to prevent the slip ratio from exceeding a prescribed limit.
The tire gripping force is also important in ensuring an adequate lateral force for each tire when the vehicle is making a turn. If the maximum available tire gripping force is insufficient for the cornering movement of the tire as often the case when the road surface is frozen or otherwise slippery, the tire may excessively slip sideways, causing the vehicle to spin or drift out in extreme cases.
When a linear actuator interposed between a wheel and a vehicle body is either extended or retracted at a certain acceleration, a corresponding inertia force is produced in the sprung mass and the unsprung mass. The reaction of such an inertia force may be used to increase the contact load of the road wheel or the gripping force of the tire. Therefore, when this concept is implemented, it is possible to increase the tire gripping force which may serve as the braking force, the traction force and/or the lateral force of the wheel. In particular, the inventors have recognized an optimum result can be achieved if the contact load of the wheel is increased when the slip ratio of the wheel is about to exceed a threshold level beyond which the tire grip force starts diminishing.